Organometallics, compounds containing carbon-metal bonds, tend to be compatible with polar polymers such as PVC. Most organometallics have disadvantages: toxicity (alkyl lead, zinc and mercury compounds), violent reactivity to air or water (organo-aluminum and titanium compounds), or high reactivity towards polar substrates (silanes). An exception is the case of mono- and dialkyltin derivatives. In a typical stabilizer, tin, in the (IV) oxidation state, is covalently bound to either one or two alkyl groups, and, correspondingly, either two or three reactive ligands. The latter are capable both of displacing labile chlorine and scavenging HCl. The resultant mono- and diorganotin compounds form a group of valuable stabilizers, principally for rigid PVC, but with some flexible PVC use. The generalized stabilizer formula is R.sub.2 SnX.sub.2 or R.sub.2 SnX.sub.3.
In the first generation of tin stabilizers to appear in the late 1930's in the United States, the R-group used in the above general formula was n-butyl, because of the availability and relative low cost of n-butyl chloride, and its suitability in the Grignard reaction with magnesium, as follows: EQU R--Cl+Mg.fwdarw.R--Mg--Cl (1)
The resultant Grignard reagent reacts rapidly with tin halides, replacing chloride with the R-group. In practice, tin tetrachloride was converted to tetrabutyltin, originally with butylmagnesium chloride, more recently with the corresponding aluminum trialkyl. EQU SnCl.sub.4 +4R--MgCl.fwdarw.R.sub.4 Sn+4MgCl.sub.2 (2) EQU 3SnCl.sub.4 +AlR.sub.3.fwdarw.3R.sub.4 Sn+4AlCl.sub.3 (3)
The resultant tetraalkyltin is then reacted further with tin tetrachloride, causing disproportionation into a range of alkyltin chlorides, which are separated by fractional distillation. EQU R.sub.4 Sn+SnCl.sub.4.fwdarw.RSnCl.sub.3 +R.sub.2 SnCl.sub.2, etc. (4)
The disproportionation process and fractionation can be controlled to yield mixtures of almost entirely mono and diorganotin chlorides, ranging from an equilibrium 65/35 di/mono ratio to pure mono- or di-, depending on reaction conditions. These intermediates are then reacted with carboxylic acids or with ligands containing mercaptan groups to yield actual stabilizers.
Dimethyltin dichloride intermediate is now produced directly from tin and methyl chloride. EQU Sn+2CH.sub.3 Cl.fwdarw.(CH.sub.3)SnCl.sub.3 (5)
Similarly, monomethyltin trichloride is synthesized from stannous chloride: EQU SnCl.sub.2 +CH.sub.3 Cl.fwdarw.CH.sub.3 SnCl.sub.3 (6)
These methods are cost effective and, in addition, directly yield intermediates without the need for fractional distillation of a mixture. However, these alkyltin chloride intermediates are corrosive, require glass lined reactors, and have undesirably toxic properties.
It would be very advantageous to eliminate the need to produce alkyltin chloride intermediates in making organotin stabilizers. New methods are also desired for producing organotin compounds to overcome the disadvantages of known methods.